In the process for fabricating a semiconductor, an adhesive tape is applied to a wafer, when the wafer back side is to be polished, so as to protect the wafer surface. In an adhesive tape cutting device to be used for such process, as disclosed in Japanese Patent Laid-Open No. 64-43458, there is applied to a positioned wafer an adhesive tape which is larger than the wafer. After this, the adhesive tape is cut along the outer circumference of the wafer to remove the portion protruding radially beyond the circumference of the wafer.
A silicon wafer generally has a disc shape, and a portion of its outer circumference is cut away straight, (called the "orientation flat" or "O.F."). In the aforementioned adhesive cutting device, as disclosed in Japanese Patent Laid-Open No. 61-254304, a wafer positioning method uses a roller to detect the O.F. of the wafer. After the wafer has been positioned with reference to the O.F., a cutting blade is linearly moved along the O.F. and then along the arcuate portion in the outer circumference of the wafer to cut away the adhesive tape along the outer circumference of the wafer.
However, the devices thus constructed have the following problems.
Specifically, some wafers are formed with not only a first O.F. with a larger notch, but also second and third O.F.s with smaller notches. Another wafer may be chipped partially in its outer circumference. When it is intended to cut the adhesive tape applied to such a wafer, what is detected by the wafer positioning method described above is only the first O.F., but not the wafer contour such as other O.F.s or chips. When a cutting blade is moved along the wafer circumference to cut the adhesive tape, there arise problems that the inertia of the cutting blade leaves the adhesive tape uncut at the second and/or third O.F.s, and the cutting blade cuts into the chipped portion so that the blade itself and/or the 15 wafer are broken.
If the wafer has a varying outer diameter, on the other hand, the conventional method using the roller fails to catch the center position of the wafer. When the O.F. is to be adjusted to some reference, the conventional devices are difficult to adjust.
In addition, the optical sensor used in these devices has a wide luminous flux, so the method of using the optical sensor is not reliable for the detection when the portion of the wafer to be detected is slightly notched.
If, in this case, the center of the wafer is eccentric to the center of rotation, the problem of the devices' inability to recognize the notch or the eccentricity is left unsolved by using a sensor having a thin optical axis.